The invention relates to hollow sections which are produced by pressing on a bridging tool and which have an electric conductivity of at least 95% IACS (International Annealed Copper Standard).
The demand for constantly higher power per unit of volume on the part of electric machines, induction furnaces, magnetic coils and similar units requires the use of special material and constructions for practically all construction parts, particularly current-carrying conductor elements. Due to the high current load on the conductors, heating takes place to such an extent as to make increased cooling necessary in order to reduce the losses, keep the thermal imbalances small and bring the changes in length, which frequently have a very disagreeable effect, within controllable limits. Indirect cooling of such conductor elements is no longer sufficient for such high power density. For these reasons a shift has taken place towards direct cooling of copper conductors, for instance internal conductor cooling. For this purpose, specially developed hollow sections have been designed. A number of requirements must be satisfied by these hollow sections. First of all, the hollow sections must be absolutely sealed, since they are generally cooled with hydrogen gas or a liquid, for instance water. Furthermore, high mechanical strength is required in order to prevent the hollow section from being deformed under the action of high centrifugal forces. High electrical condutivity is desired to prevent excessive heating of the conductor sections.
Such hollow sections are described in "Prometall," 1962, pages 678 to 683. Continuous casting over a bridging tool is desired as being the most advantageous manner of manufacturing such hollow sections. The heated copper is pressed around the bridge, which has one or more mandrels or mandrel extensions which form the hollow duct or ducts. The two streams of metal are combined in the region of the die and welded together there uner strong pressure. After the pressing, the hollow sections are brought, in one or more steps into the desired final shape, bright annealing being possibly effected between the individual steps.
The said article requires electrolytic copper, oxygen-free copper or else a copper-silver alloy as material for such hollow conductors. The copper of most favorable price and which at the same time also has the highest condutivity is commercial electrolytic copper. Its oxygen content is about 0.02 to 0.04%. This high oxygen content can lead to the dreaded hydrogen disease which is of importance, in particular, for welding and soldering work. Oxygen-free copper, i.e. copper having no oxygen bound to copper, has an oxygen content which is about 10 times less, is insensitive to hydrogen embrittlement and has a somewhat higher softening point but, in general, a conductivity which is about 1% less.
Oxygen-free deoxidized grades of copper of high electrical conductivity are standardized by DIN 1708. The copper content is at least 99.90%; the deoxidizing agent, which is ordinarily phosphorus, is present in an amount of about 0.003%. Upon fabricating these types of copper by means of bridge tools, defects can occur in the region of the streams of material which are to be welded together. As cause of these defects there enter into consideration primarily enrichments of hydrogen in the region of the weld seam, where a hydrogen-disease structure is formed upon intermediate and/or final annealings in a hydrogen-containing atmosphere, and this may lead to the formation of cracks. The oxygen passes into the weld seam, for instance, via the oxides adhering to the surface of the block, which have oxides are upon the heating or the bringing of the block to the press, particularly on its end surface.